Generally, the most commonly encountered fluorescent lamps are the so-called "preheat" and "rapid-start" types of fluorescent lamps. In the "preheat" type of lamp, heater current flows through the lamp electrode during lamp ignition. Thereafter, an external voltage sensitive starter opens the electrical circuit to the lamp electrode and heater current flow is discontinued. The "rapid-start" type of fluorescent lamp normally has a constant flow of heater current through each electrode not only during ignition but also during operation of the lamp. However, heater current flow during operation, as in the "rapid-start" lamp, is lost power which undesirably reduces the operational efficiency of the lamp.
Numerous suggestions have been made for enhancing the efficiency of "rapid-start" type fluorescent lamps. For example, U.S. Pat. Nos. 4,052,687; 4,097,779; 4,114,968; 4,156,831 and 4,171,519 which are all assigned to the Assignee of the present application, provide numerous configurations for enhanced operation of "rapid-start" type fluorescent lamps. Primarily, each provides a thermally responsive circuit breaker suitable for discontinuing heater current during operation of the fluorescent lamp.
Although the above-described fluorescent lamp and circuit breaker structures have numerous advantages, it has been found that there are problems with the known configurations. More specifically, the known rapid start fluorescent lamps utilizing a circuit breaker therein normally employ a circuit breaker of the type having a substantially U-shaped bimetal switch. This U-shaped thermally responsive bimetal switch includes a pair of leg portions of substantially uniform length. One of the leg portions is welded to an electrical conductor passing through the glass envelope of the circuit breaker and the other leg portion makes contact with another electrical conductor passing through the glass envelope.
While the above-described circuit breakers having a U-shaped bimetal switch with a pair of leg portions of equal length have been an improvement over other known structures, it has been found that the leg portion which is attached to one of the electrical conductors tends to bend, upon the application of heat thereto, prior to the bimetal moving away from the other electrical conductor whereat a switching contact is present. As a result of this undesired bending of the bimetal, it was determined that the opening temperatures of the bimetal switch vary by several degrees centigrade depending upon the amount of flexure occurring along the leg portion fastened to the electrical conductor. Moreover, this undesired bending of the leg portion of the U-shaped bimetal caused a shift in the location at which the opposite leg portion contacted the other electrical conductor. This variation in contact point was also detrimental to a uniform operation of the switch at a given temperature.